A student investigates the breakdown of fats in milk by lipase. Four test-tubes labelled A to D are set up. The table shows the contents of each test-tube. In which test-tube will the contents become acidic most quickly?

Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases. Lipase break down fats into monoglyceride and diglycerides (fatty acid and glycerol).
Test tube A has milk, bile and lipase. So, the fat in the milk will be broken by lipase and bile, imparting acidity to the solution.
Test tube B does not contain lipase so there will be no breakdown of fats.
Test tube C does not have bile as well as lipase, again there will be no breakdown of fats.
Test tube D contains lipase so it will break down fats into smaller subunits, imparting acidity. However due to absence of bile further breakdown of fat subunits is not possible. Therefore, this solution will be less acidic than the solution in test tube A.
Hence, in test-tube A the contents will become acidic most quickly.